Signaling



1929- J. F. FARRINGTON I. 1,724,917

' SIGNALING Filed July 51, 1925 3 Sheets-Sheet l Tunz'n 72mm ,eeaciarrqe1929- J. F. FARRINGTON 1,724,917

S IGNALING Filed July 51, 1923 3 Sheets-Sheet 2 was;

Tum Aeac nce v /n venior: John fT/rr/ngfon J. F. FAFQRINGTON SIGNALINGAug. 20, 1929.

Filed July 51, 1923 F. Network .6, y 5 F/lfer Y 40 L 45 v 4? 3Sheets-Sheet 3 HM 7 F w fly. 65.

hvenfon- Patented Aug. 20, 1929.

UNITED STATES PATENT OFFICE.

JOHN F. FARRINGTON,

OF FLUSHING, NEW YORK,'ASSIGNOR TO WESTERN ELECTRIC COMPANY,INCORPORATED, 015 NEW YORK, N. Y., A CORPORATION OF NEW YORK.

SIGNALING.

Application filed July 31,

This invention relates to signaling systems and as illustrated in theembodiments herein described, is particularly adapted for use in radiotelephone systems. a

A portion of the subject matter described and claimed herein isdisclosed in applicants copending application Serial No. 326,986 filedSeptember 27, 1919.

Duplex working of radio or other high frequency systems has previouslybeen a matter of some difliculty because of the excessive influence ofthe local transmitter upon the local receiver at each station.

It has often been the practice in desi ming radio systems, to provide aswitch for switching an antenna system from a transnitting to areceiving set so that transmission and reception could be carried onalternately. It has also been suggested to provide a transmitterconnected to an antenna by an arrangement whereby a minimum amount ofthe transmitted energy is impressed upon the receiving set. In accordance with the principles of this invention, however, it is not attemptedto completely prevent the energy of the outgoing carrier frequency fromacting upon the local receiver. A certain amount of energy of theoutgoing carrier frequency is utilized in the receiving set. This causesamplification of the received signals and also produces a side toneeffect similar to that in an ordinary telephone substation. The loudnessof the out.- going signals or the effect thereof upon the localindicating instrument, will generally be of approximately the samedegree of magnitude as that of the lncoming signals.

These results are accomplished by causing the received energy and acertain amount of energy of the outgoing carrier frequency to becombined and detected, thereby producing a local auxiliary carriercurrent wave. This current of auxiliary carrier frequency will bemodulated or varied in amplitude in accordance with both the out oingand the incoming signals and by detecting it a wave will be producedfrom which may be derived both the incoming and the outgoing signals.The auxiliary carrier frequency may be equal either to the sum orthedifference of the principal carrier frequencies, but since manypractical advantages are attached to using the difference frequencyrather than the sum frequency, this is the one which may mostconveniently be utilized.

The amplitude of 1923. Serial No. 654,823.

the auxiliary carrier frequency current will be proportional to theproduct of the amplitudes of the local and the distant. carriers. Thus,assuming the same percentage of modulation at each station, the signalheard in the local receiver will be of approximately the same intensityWhether the local or distant station is transmitting. in order to obtaina maximum indication from the distant. station, it is consequentlydesirable to allow sufficient energy of the local carrier wave frequencyto enter the receiving circuit to load the detector or the rectifyingdevice with approximately the maximum energy input which it is capableof handling without material signal distortion. The local energy is thusutilized as a desirable amplifying and selecting agent during thenon-transmitting periods.

The object. of the invention, broadly stated, is, therefore, to provideimproved methods of and means for duplex signaling, and morespecifically for radio signaling.

A particular object is to provide a duplex system wherein the locallygenerated carrier wave for the outgoing signals aids in selecting andamplifying the incoming signals. The effects of interfering waves onradio systems are thereby, to a considerable extent, overcome.

A further object is to provide a system in which the transmittingoperator will at once become cognizant of an inoperative condition ofhis transmitting or receiving system or that of the distant transmitter.The absence of side tone effect will warn the operator that theapparatus is not working properly.

A further object is to provide improved methods of duplex communicationin which a single element such as an amplifier or oscillator of atransmission circuit may function usefully as an element of the receivercircuit. This results in a desirable simplification of duplex signalingsystems which are capable of transmitting efficiently and re ceivingselectively.

A further object is to simplify systems for producing modulated Wavesand relates especially to producing modulated Waves from which theunmodulated carrier component has been suppressed by means of a singlevacuum tube arrangement which is particularly adapted for use in duplexcommunication systems. v

A still further object is to reduce the num- Cir her of vacuum tubesen'iployed in duplex com munication systems.

Further objects of the invention will be apparent to those skilled inthe art from a perusal of the following detailed description withreference to the accompanying drawings,

wherein Fig. 1 represents a system in which an amplifier tube of thetransmitting circuit functions as adetectorfor the receiving circuit;Fig. 2 represents a system in which a high frequency oscillator of thetransmission system functions as a detector of the receiving circuit;Fig. 3 is a modification of Fig. 2 in which low frequency balance isemployed to reduce the side tone introduced into the receiving system;Figs. land 5 are circuit diagrams of systems corresponding respectively.to the arrangements of Figs. 2 and 3 but male ing use of bandpassfilters where the arrangements of Figs. 2 and 3 employ coupled tunedcircuits; Fig. 6 is a circuit diagram included for the purpose ofexplaining a single tube carrier suppression modulating system ;'Figs. 6and 6 indicate specific aJi-angements of impedance elements which may beemployed in Fig. 6; Fig. 7 represents an arrangement whereby thecompleteness of carrier suppre sion may be estimated and the circuitacct.- rately adjusted to increase the completeness of the carriersuppression; Fig. 8 is a simple transmission system employing a singletube carrier suppression modulator; and Fig. 9 is a circuit diagramillustrating the utilization of a circuit, such as that shown in Fig. 8,in a partial carrier suppression difference frequency duplex telephonesystem.

Description of Fig. 1. Referring specifically to Fig. 1, outgoing speechsignal waves 8 from the microphone circuit 1, are in pressed with a highfrequency carrier wave f from the source 2, which, for example, may be afrequency of 500,000 cycles per second, on the input circuit of amodulating tube or other equivalent modulator A wave of frequency fmodulated by speech 3 is thereby produced in the output circuit f andimpressed by means of the transformer 5 upon the amplifier 6, which ispreferably of the thermionic type. The output circuit of the amplifier 6includes a circuit A which is preferably tuned to the mean frequency ofthe currents resulting when current of frequency f, is modulated byspeech waves 5 The frequency of this modulated current is hereindesignated by the expression f e This symbolic manner of designating amodulated wave will be used throughout this specification. A modulatedwave so designated may or may not contain an unmodulated carriercomponent. iln the operation of the various systems of the invention,can ,cept in the operation according toone method of the system of Fig.9, the transmitted 1nodulated wave does includes such unmodulatedcarrier component, as is necessitated by the lack of a local homodynesource in such sys tems.

A. circuit B is tuned to the frequency f I which is the mean frequencyof the outgoing modulated wave f s, in order to prevent the outgingwaves which are impressed upon the antenna 7 from being shunted to anymaterial extent through the shunt circuit 8 in which the circuit B isplaced. Any suitable reactance means 9, such as in the present instancea coil 10 shunted by a condenser 11, is provided in the shunt branch 8as a tuning reactance to give the antenna system a degree of freedomequal to the mean of the frequencies f is which represents the modulatedwave received from a communicating station. A tuned circuit C isprovided in the output circuit of the amplifier 6. This circuit is tunedto a frequency equal to the difference between the frequencies 7, and fThus, if frequency f, equals 500,000 cycles and f eq .als 530,000cycles, circuit C will be adjusted to be resonant at 30,000 cycles.Owing to the detecting or demodulating action of the amplifying tube 6which has its anodecathode circuit in series with circuit C, there willoccur in circuit Coscillations having component with a frequency of30,000 cycles and a wave of this frequency modulated in accordance withthe speech signal 8 or the signal a or by both of themin case both arebeing simultaneously transmitted. This is represented by the legend f fi 2 adjacent the circuit C. A circuit D tuned to the same frequency asthe circuit C, is coupled thereto, preferably loosely. The circuit D isserially included in the input circuit of a thermionic or other suitabledetector 12, whose output circuit includes a suitable indicating devicesuch as a telephone receiver 13, shunted by a condenser 1%.

Operation of Fig. 1.The greater part of the high frequency energy whichis being transmitted will pass through the circuit C without producingmuch effect in circuit D. That part of the unmodulated outgoing energywhich is impressed upon the circuit C will perform the useful purpose ofaiding in the selective reception and amplification of the incomingsignal. Since the amplitude of the auxiliary or intermediate frequencyimpressed upon the detector 12 will have a value dependent upon theamplitudes of the carrier frequencies of the two stations, the signalheard in the local receiver will consist of both the local and thedistant signal and these will be of the same order of loudness. Thelegened s 3 adjacent the receiver 13 indicates that both signals will beheard in this instrument. I

The use of the letters f and a with appropriate-subscripts are appliedto the remaining figures of the drawing with meanings similar to thosein connection with Fig. 1.

Description of Fig. 2.The transmitting system shown in Fig. 2 comprisesan antenna 7 to which is connected a vacuum tube 15 in a well-knownmanner as illustrated, whereby oscillations are produced in the antennaat a frequency approximately equal to the natural frequency determinedby the series arrangement comprising the capacity of the radiatingmember 7, the inductance of the coil 16, and the capacity of thecondenser 17. Space current for the tube 15 is provided by the source18, through the speech frequency choke coil 19, the circuit 20, and thehigh frequency choke coil 21. The choke coil 21 functions todiscriminate against both the incoming and outgoing high frequency wavesand therefore tends to keep them out of the circuit 20. Shunted aroundthe source 18 and the coil 19 is the anode-cathode path of the vacuumtube 22. The input circuit 23 of the tube 22 is coupled to themicrophone circuit 24: which represents any suitable source of speech orother signaling frequency electrical waves or variations. The productionof speech waves in the circuit 24 will cause a corresponding variationin the impedance of the tube 22, which will in turn cause acorresponding variation in the current supplied to the tube 15 owing tothe choking action of the coil 19. This will cause the high frequencywaves generated in the antenna by the oscillating tube 15 to vary inamplitude in accordance with the waves produced in the circuit 2%.

Incoming signal waves will be impressed upon the antenna 7 Owing to thedetecting action of the tube 15 upon both incoming and outgoin highfrequency waves, a variation of potential difference will be producedacross the anode and the cathode of the tube 15 having frequencies equalto the differences between the outgoing and incoming frequencies. Thetuned circuit 20 will be adjusted to be resonant to these differencefrequencies. In selecting the modulated auxiliary carrier frequency wavethe circuit 20 functions in the same manner as the circuit designated Cin Fig. 1.

A circuit 25 comprising a variable condenser and a variable inductancemay be connected in parallel to the circuit containing the source 18 andthe coil 19. The circuit 25 may be most advantageously tuned to beresonant at the auxiliary carrier frequency. This prevents a loss ofenergy of the auxiliary carrier frequency due to the voltage drop acrossthe tube 22.

Coupled to the circuit 20 is the circuit D which is tuned to theauxiliary carrier frequency. The circuit D is in the input cir cuit ofthe detector 12 in the output circuit of which is the receiver 13. Thebranch circuit 8 which is connected to the antenna, is intended to givethe antenna one natural period equal to the period of the incoming wavesfrom the distant station which are represented by the legend f s Ananti-resonant circuit B is serially included in the branch 8 and tunedto the transmitting carrier frequency f, in order to prevent current ofthat frequency from passing through the branch 8. Any suitable tuningmeans 10 is also provided in the branch 8 whereby the said branch may begiven any desired reactance which is necessary to tune the antenna forthe incoming waves.

An important feature of this arrangement is that the frequency of thewaves being transmitted is determined by and can be varied by theadjustment of the coil 16 and the condenser 17 After establishing theoutgoing carrier at the desired frequency, the tuning reactance means 10may be varied within considerable limits without producing a materialvariation of the waves being generated by the tube 15 in the antenna.

Operation 0 f Fig. Q.-When signals are being transmitted from the localstation there will be radiated from the antenna 7, a modulated carrierwave f s which is available for combination with the incoming signalingwave 7, is, to cause the transmission through circuits 20 and D of thewave f f,i

2 This produces in the receiver 13 either or both of the signals 8, and8 depending upon whether one or both are at that instant beingtransmitted. lVhen no signals are bein transmitted, of course, only theunmodulate component of the outgoing carrier combines with the incomingwave to effect amplification thereof.

Description of Fig. 3.Fig. 3 represents a system in which the elements7, 16, 17, 15, 18, 19, 20, 21, 22, 23, 24, 12, 13, 14 and D are similarto and function in the same way as the correspondingly designatedelements in Fig. 2. In the arrangement of this figure the antenna istuned for the outgoing carrier wave frequency and hence, incomingsignals will be received rather inefficiently. However, a circuitcontaining elements similar to the circuit 8 of Fig. 2 may be providedif it is desired to provide an antenna having a degree of freedomcorresponding to the frequency of the incoming waves. In Fig. 2 thevoltage of speech frequency which is supplied to the oscillating tube 15from the tube 22 is impressed serially across the circuit 20. In somecases, this results in objectionable transients being set up in circuit20 which cause interference with the operation of the system. In orderto avoid this in the arrangement of Fig. 3, the circuit 20, whichcorresponds to the circuit C of Fig. 1 is balanced for speechfrequencies. The lead from the plate ofthe tube 22 is connected to anintermediate point 27 of R of the circuit from the point 27 through thework is so adjusted that the time constant oscillator 15 to ground isequal to the time constant gof the circuit from point 27 for theoscillating tube and comprise an inductance in series with a parallelcombination of a resistance and a condenser.

The currents of the intermediate carrier frequency in the circuit Dinstead of being impressed directly upon detector 12 are first amplifiedby means of a suitable amplifying device 31 and then passed through anysuit ablefilter 32 to the detector 12. The filter 32 1s deslgned to passonly modulated auxiliary I s carrier frequencies f f i 9 m 1 7,, .2 rhegeneral opei atlon of this system iollows obviously from the describedoperation of the system shown in Fig. 2. Balancing the circuit forspeech frequencies will reduce the number and amplitude of disturbingtransients introduced in the receiving circuit and will reduce theamplitude of speech frequency side tone directly introduced into thereceiving circuit. 7

Description of Fig. .4.Fig. 4-is similar to Fig. 1 but includesadditional means to increase the selectivity and to reduce the sidetone. The tuned circuits C and D. of Fig. 1 are replaced by a band passfilter 33 of a well-known type. The function of the filter 33 is toselect a band of frequencies equal in width to the modulated waveresulting from the combination of the incoming and outgoing waves and tosubstantially suppress waves of other frequencies. The band pass filter33 is connected to the input circuit of an amplifier 34. T o the outputcircuit of the amplifier. 34 is connected a band pass filter 32 similarto filter 33 or otherwise constructed may be desired.

The filter 32 supplies the input circuit of an amplifier which in turnsupplies the detector 12. It will thus be seen that the intermediatefrequency which may be equal either to the sum or the difference of theoutgoing and incoming carrier waves is first selected by the band filter33, amplified by the amplifier 34, again selected by the filter 32 andagain amplified with the amplifier 35. Such an ar rangement is highlyefficient in selection and amplification. Additional stages ofamplification or additional selective means may be inserted in thecircuit at any point.

The operation of the circuit of Fig. 4 follows obviously from thedescription of the operation of Fig. 1. However, it may be not-ed thatthe terminal section of the filter 33, which is serially included in theplate-filament circuit of the tube 6 should offer a reasonably lowimpedance to the incoming and outgoing carrier waves.

Description of F ig. 5.Fig. 5 is a modified form of Fig. 3 in whichcorresponding elementsare identified by similar reference characters.The band filter 33, amplifier 34E, band filter 32, amplifier 35 anddetector 12 function in the circuit of Fig. 4. By a tap 27 upon the coil23 a balance of speech frequencies is obtained whereby the amount ofspeech frequency voltage introduced into the filter 33 is reduced.

Description of Fig. 6'.-Fig. 6 is a circuit diagram illustrating theprinciple of operation of a single tube carrier suppression modulatorcircuit which, among other uses, is useful in difference frequencyduplex systems. In (3, the input circuit of a modulating tube 40 issupplied with carrier waves from a source ll and signaling waves from asource 12. The plate-filament circuit of the tube 40 is connected to areceiving instrument 43 through a filter 4-1. An impedance network 4-5is connected between the grid and the plate of the tube 40. The usualbatteries and the choke coil 46, together with a stopping condenser 77are employed in the circuit as indicated. In a circuit'such asillustrated in Fig. 6, it is possible to suppress from the outputcircuit current of the car rier frequency supplied by source 41 andcurrent of the signaling frequency supplied by source 42.

The physical principles, on which 6.18 operation of the present circuitare, in a broad sense, based, are explained in the description of Fig. 1of U. S. patent to Nichols 1,558,909, October 2?, 1925. In the patentit'is shown that a three-element electric discharge device may beconsidered a virtual source, adapted to impress on its output circuit anelectromotive force similar to that impressed on its grid but oppositein phase; or, from another point of view, as a means for transmitting animpressed electromotive force wave through a circuit of impedance? theplatefilan'ient impedance) with an amplification ,u. and a reversal ofphase. It is the function of the network, connected between the'grid andplate, to transmit to the same points in the out-put circuit anelectromotiv forc equal and opposite to the electromotive force from thevirtual source. In order to transmit through the network 45 anelectromotive force which will be equal to that transmitted through thedevice, the impedance of the network must satisfy the relation Z Sinceimpedance R is substantially a pure resistance, the impedance Z must beeffectively a pure resistance, as it is in the Nichols circuit in whichthe network comprises a pure resistance and a circuit, consisting ofreactive elements, tuned to the frequency at Which balance is to beeffected. Applicant has improved the Nichols arrangement to the extentof balancing out all the electromotive force waves impressed on thegrid-filament circuit, by providing a network whose impedance issubstantially a pure resistance for all frequencies. He is therebyenabled to accomplish with a single tube substantially the same resultsas could otherwise be accomplished by the use of a push-pull arrangementof two tubes as illustrated, for example, b U. S. patent to Hartley1,419,562, June 13, 1922.

The above relation holds only for frequencies applied to the inputcircuit and not to those frequencies which result from the curvature ofthe tube characteristic and which appear in the output circuit. Amongsuch frequencies resulting from the curvature of the tube characteristicare modulated components and multiple frequencies. With the value ofimpedance corresponding to the above mentioned condition, if the voltageamplitude of waves of carrier frequency f from source 41 impressed onthe grid circuit is represented by 6 and if the source 42 is a source ofsignaling frequency 8 which ap plies to the tube 40 a voltage ofamplitude 6 the output voltage of frequency 2f will be isfy the equationZ for the frequencies f ands, it may approximate a pure resistance.

Fig. 6 discloses one arrangement in which a large condenser 47 and avariable resistance 48 are connected between the grid and the plate ofthe tube 40. In Fig. 6 the variable resistance 48 is placed in serieswith a source of electromotive force 49 so adjusted as to balance theplate-filament battery of the tube 40. The source 49 has its positiveterminal connected to the plate.

The suppressing effect of such an arrangement as illustrated in Fig. 6may be verified experimentally for audible frequencies by an arrangementsuch as that of Fig. 7. A source 50 supplies waves of any frequency forwhich the circuit is to be tested. This current is supplied to the tubeinput circuit through the transformer as shown and the telephonereceiver 43 is connected to the output circuit. The receiver 51 isdirectly connected to the source 50. By listening at receivers 43 and 51simultaneously and adjusting the resistance 48, the particular value ofthe resistance may be found for which the only tone heard in thereceiver 43 corresponds to a harmonic or multiple frequency of the toneheard in receiver 51. When speech currents are supplied from source 50,the condition for balance is not so exactly defined as with a singletone because the double frequency of speech in general constitutesintelligible sounds. For speech frequencies, therefore, the actualadjustment is made at that point at which the operator concludes thatthe sounds produced by receiver 43 consists principally of harmonicfrequencies.

Decripzc'on of Fig. 8.A simple transmission circuit involving a singletube carrier suppression modulator is shown in Fig. 8. Source 41supplies waves of carrier frequency and source 42 waves of signalingfrequency to the modulating tube 40. These elements are shown asserially included in the input circuit, which should be taken toindicate diagrammatically any known method of supplying waves of thesefrequencies to the input circuit by means of inductive, capacitative, orresistance couplings arranged in series or parallel. The network 45consists of the large condenser 47 and adjustable resistance 48. Asmentioned above, the amplitude of the modulated components may be madelarge as compared to the amplitude of the audible signaling frequenciesby making the carrier wave voltage large with respect to the signalingwave voltage. This results in a relatively large amplitude harmonic ofthe carrier wave frequency, which may be suppressed by the low pass orband pass filter 44, which suppresses multiple frequencies of thecarrier wave source and may also suppress multiple frequencies of thesignal wave source. If desired, the filter 44 may have its cut-off limitat such a point as to suppress one side band of the modulated wave. Thecurrents trans ferred through the filter 44 are amplified by anysuitable amplifying system, such as the amplifier 52, and radiated fromantenna 53 or otherwise transmitted or utilized as may be desired.

Fig. 9 represents a duplex system employing a single tube carriersuppression modulator such as is described in connection with *ig. 6. Inthis circuit the modulator is adjusted so that the unmodulatedcomponentof carrier frequency current is not entirely suppressed for areason which will be hereinafter stated.

In Fig. 9 the source 60 supplies waves of some convenient basefrequency, for example 50,000 cycles per second. A harmonic generator 61is connected to the source 60 and produces waves of a suitable multiplefrequency, for example 500,000 cycles per second, which are-selected byany suitable selective network 62. A. proper voltage of this frequencymay be taken off from the potentiometer 63 and applied to the inputcircuit of th modulator 40. Signaling waves to be transmitted aresupplied from the microphone circuit 64 to the-input circuit of themodulator 10, which is so adjusted as to almost but not quite suppressthe unmodulated carrier frequency. The resulting modulated waves aretransmitted through a filter ll to eliminate multiple carrierfrequencies, amplified l y the amplifier 6 and impressed upon theantenna 7 by means of the coupling coil in the tuned circuit A. Thetuned circuit B is tuned to the outgoing carrier frequency and thenetwork 10 serves to tune the antenna for the incoming carrierfrequency. If desired, a circuit connection 68 may be employed toimpress upon the input circuit of the amplifier 6 an adjustable smallcomponent of the outgoing carrier wave frequency. For this purpose theconnection 68 is provided with an adjustable tap upon the potentiometer63. There are thus radiated from the antenna 7 modulated carrier wavesof the frequencies f is where 7, represents 500,000 cycles and 8 thesignal to. be transmitted. A component of the unmodulatedcarrierfrequency may be included in the radiated energy and may be regulated toany desired amount. Let it be assumed that incoming carrier waves ofsimilar nature having the frequencies of f -8 where represents 550,000cycles, are being received. The incoming waves combine in theplate-filament circuit of the amplifier 6 with the outgoing carrierwaves and owing to the rectifying action of the tube, a differencefrequency wave results. This difference frequency or auxiliary carrierwave comprises in general side bands based upon the outgoing andincoming carrier waves. The side bands and any unmodulated carriercomponent are selected from other interfering waves by a suitable bandpass filter 65. The circuit 66, including an amplifying tube 67, isloosely coupled to the input circuit of the detector 12 to supplythereto Waves having the frequency of the intermediate or auxiliaryfrequency unmodulated carrier component. These waves are combined withthe waves transferred through the filter and the action of the detector12 is such as to reproduce speech therefrom. The

legend s 8 indicates that during transmission the outgoing signal willbe heard in receiver 13 and during reception the incoming signal will beheard and will be of approximately the same intensity.

Having described several forms of the invention, the parts, features andcombinations and methods inherent therein which are believed to bepatentable are defined in the appended claims.

,VVhat is claimed is:

1. The method of duplex signaling by means of a vacuum tube poweramplifier in a system for transmitting messages in one direction betweentwo stations by means of a carrierwave and transmitting a message in theother direction upon a carrier wave of a different frequency andutilizing at one of said stations a portion of the energy of theoutgoing carrier wave to reinforce the incoming signal by producing athird wave of frequency diff rent from each of said carrier waves andderiving a wave of signaling frequency from the last mentioned wave,which comprises combining the incoming and outgoing carrier waves insaid power amplifier to produce the said third wave and transmitting theoutgoing waves by means of said device.

2. In a signaling system comprising two stations each of which comprisesa source of signaling waves, a high frequency carrier wave source, meansfor modulating each of said carrier waves in accordance with thecorresponding signaling wave, means for combining the energy of each ofsaid carrier waves modulated at times to produce a similarly modulatedintermediate frequency car rier wave, means for deriving the signalingwaves from said intermediate frequency carrier wave, and a poweramplifier in the outgoing wave channel, said amplifier comprising saidcombining means.

3-. In a system for duplex radio signaling, a transmission channel fortransmitting outgoing waves including an electron discharge device,means for receiving incoming waves and combining them by means of saiddischarge device with energy of the outgoing wave frequency to produce awave of carrier frequency modulated in accordance with incoming andoutgoing signals, and a common transmitting and receiving antennaimpartially resonant to the mean frequency of each of the outgoing andincoming waves.

4:. In a duplex radio signaling system, common radiating and absorbingstructure adapted to resonantly oscillate at the mean frequency of boththe transmitted and received waves, an electron discharge device havingan anode-cathode path, means for impressing the waves to be transmittedupon said anode-cathode path, means for impressing the received wavesupon said path, a circuit connected to said path including selectivemeans for transferring electric waves having a frequency equal to thedifference of the incoming and outgoing wave frequencies, and means fordetecting and indicating the selected waves.

5. In a duplex high frequency telephone system, means for transmittingmessages upon one carrier wave and means for receiving messages uponanother carrier wave of different frequency, means for combining the twocarrier waves to produce an auxiliary carrier Wave, a circuit in whichthe three carrier Waves are present, and a band-pass filter forselecting from this circuit the auxiliary carrier Wave and supplying itto a receiving circuit.

6. In a duplex radio system for transmission and reception on differentcarrier waves, a transmitting circuit and a receiving circuit, anelectron discharge device for relaying oscillations to be transmittedand for combining incoming oscillations with oscillations to betransmitted to produce combination frequency oscillations, a circuitassociated with said device in which said combination frequencyoscillations are produced, and a bandpass filter connected to saidcircuit for selecting the combination frequency oscillations from thereceived and transmitted carrier frequency oscillations.

7. In a duplex carrier Wave system in which transmission is accomplishedon one carrier wave frequency and reception is accomplished on adifferent carrier Wave frequency, the combination of a circuit adaptedto carry waves corresponding to transmitted and received signals, atransmitter circuit and a receiver circuit, comprising a Wave distortingdetector, both associated with said first mentioned circuit, and aband-pass filter associated with the receiver circuit to select from andsupply to said receiver circuit Waves resulting from the combination ofincoming and outgoing carrier waves to the exclusion of other Waves.

8. In a signaling system, means for combining incoming and outgoingcarrier waves to produce an auxiliary carrier wave, in combination withmeans for first amplifying and then selecting the auxiliary carrier Waveenergy, said selecting means comprising a band filter.

9. In a signaling system, means for combining received energy withlocally generated energy to produce an auxiliary carrier wave, incombination with means for first amplifying and then selecting theauxiliary carrier wave energy, said selecting means comprising a bandfilter.

In witness whereof, I hereunto subscribe my name this 27 day of July A.D., 1923.

JOHN F. FARRINGTON.

